1. Field of the Invention
An exemplary embodiment of the invention relates to an electroconductive polymer composition, a method for producing the same, and a solid electrolytic capacitor using an electroconductive polymer composition.
2. Description of the Related Art
Electroconductive polymer materials are used for electrodes of capacitors, electrodes of dye-sensitized solar cells, electrodes of electroluminescence displays and the like. For such electroconductive polymer materials, the polymer materials by polymerizing pyrrole, thiophene, 3,4-ethylenedioxy thiophene, aniline, or the like are known. In particular, the electroconductive polymer materials formed using the electroconductive polymer suspension solution using a polyanion as a dopant, which are disclosed in Patent documents 1 and 2, show high electroconductivity, and thereby generate a great interest in the fields of electronic materials and the like.
Patent document 1 relates to a solution (dispersion) of polythiophene and a method for producing the same, and to the use of a salt to an antistatic treatment of a plastic molded article. Patent document 2 relates to a water dispersion of a complex between poly(3,4-dialkoxythiophene) and a polyanion and a method for producing the same, and to a coating composition containing the water dispersion and a coated substrate having a transparent electroconductive film formed by applying the composition. The above-mentioned Patent documents 1 and 2 both relate to a polythiophene consisting of structural units of 3,4-dialkoxy thiophene, a solution of a polythiophene containing a polyacid ion derived from a polystyrene sulfonic acid, and a method for producing the same, and to the use of a salt to an antistatic treatment of a plastic molded article.
In the above-mentioned method for forming an electroconductive polymer composition by using an electroconductive polymer suspension solution, there is an advantage that an electroconductive polymer composition having high electroconductivity can easily be obtained only by drying the solution. Thus, the method is progressively applied to a solid electrolytic capacitor using the electroconductive polymer composition as a solid electrolyte, instead of the method for forming an electroconductive polymer by conventional chemical oxidative polymerization or electropolymerization method. If an electroconductive polymer composition is used as a solid electrolyte of a solid electrolytic capacitor, an electroconductive polymer layer having a thickness of a certain level or more on a dielectric film is necessary to suppress a leakage current (hereinafter referred to as LC) when the capacitor is implemented to a substrate as described in Patent document 3. Patent document 3 relates to a method for forming a polypyrrole layer of 5 μm or more as a solid electrolyte of a solid electrolytic capacitor by using chemical oxidative polymerization method.
When a capacitor is implemented to a substrate, thermal stress of an outer packaging resin is applied to an oxide film and the oxide film damages, and thereby LC may increase. However, if an electroconductive polymer layer is formed on an oxide film, the oxide film locally produces heat by LC increasing, and the electroconductive polymer layer formed thereon is thermally oxidized and in due course has a loss of electroconductivity. As a result, an electric current is interrupted to defected part of the oxide film, and thereby LC comes back to a normal level even if it temporarily increases.
However, if there is no electroconductive polymer layer on an oxide film, naturally, the above-mentioned insulation restoration is not taken place, and thereby LC increasing is actualized and comes to be a problem. Also, even if an electroconductive polymer layer is formed on an oxide film, if the film is thin, there is generated a trouble that LC increases after substrate implementation, and enough film thickness must be secured.
The cause of LC increasing after substrate implementation in the case of an thin electroconductive polymer layer is not clear. However, as one of the causes, LC is thought to increase due to the above-mentioned cause because an electroconductive polymer layer slips by thermal stress at the time of substrate implementation and there is generated a microscopic area in which an electroconductive polymer layer is not formed on an oxide film.
The problem regarding LC of the above-mentioned electrolytic capacitor exists not only in the electroconductive polymer obtained by chemical oxidative polymerization but also in the electroconductive polymer obtained by drying an electroconductive polymer suspension solution.
As a simple method for forming on an oxide film of an electrolytic capacitor an electroconductive polymer composition obtained by drying an electroconductive polymer suspension solution, there is performed a formation method which has immersing a valve metal powder sintered body on which an oxide film is formed or a valve metal body on which etching treatment is performed to an electroconductive polymer suspension solution and thereafter drying.
In the case of this method, if the electroconductive polymer suspension solution does not have a certain level of viscosity, only a small amount of the electroconductive polymer suspension solution can adhere to a valve metal powder sintered body on which an oxide film is formed or a valve metal body on which etching treatment is performed, and thereby there is a problem that only a small amount of the electroconductive polymer is formed and the electroconductive polymer layer comes to be thin. In particular, the electroconductive polymer layer formed at the edge part is thinly formed.
It is possible to thickly form a electroconductive polymer layer by performing plural times immersing and drying steps to the electroconductive polymer suspension solution. However, the formed electroconductive polymer layer has layered formation, and thereby there is a problem that interface resistance is generated between the layers and that the obtained solid electrolytic capacitor comes to have high equivalent series resistance (hereinafter, ESR). Therefore, it is expected to develop an electroconductive polymer suspension solution having a high viscosity which makes it possible to obtain an electroconductive polymer layer having a thickness of a certain level or more by one set of immersing and drying step to an electroconductive polymer suspension solution.
As a method for increasing a viscosity of an electroconductive polymer suspension solution, there is developed a method to add a thickener as described in Patent document 4. Patent document 4 relates to an electroconductive composition containing a water-soluble electroconductive polymer having a sulfonic acid group and/or a carboxyl group, a cross-linking agent, water, organic solvent, and thickener. However, the thickener described in Patent document 4 is an insulating water-soluble polymer, and thereby there is a problem that electroconductivity of the electroconductive polymer composition is damaged if such the insulating component exists in the electroconductive polymer composition. Also, in the case of using the electroconductive polymer composition which contains an insulating thickener in the electroconductive polymer layer of the solid electrolytic capacitor, there is also a similar problem that ESR of the solid electrolytic capacitor comes to be higher.